Method of operating gas analysis apparatus

ABSTRACT

The time required for continuous gas analyzers of blast furnace top gas and the like to reach steady-state conditions after an interruption of furnace operation is greatly reduced by a process including drying the stream of gas analyzed and sealing a gas containing the analyzed constituents within the gas analysis apparatus during the period of interruption.

United States Patent 73/1; 23/254 E, 255 E, 232, 232 E; 75/60, 41

Manka July 4, 1972 [54] METHOD OF OPERATING GAS [56] References CitedANALYSIS APPARATUS UNITED STATES PATENTS [721 lnvenm" Paul Man,Pittsburgh Pi 3,531,256 9/1970 Kaczaj ..23/232 73 A I L by 3,495,4372/1970 Ester ....73/1 R 1 ss'gnee 283: P m S'eel Cmpmtmn Pm 3,520,6577/1970 Trumerman ..73/23 g a. [22] Filed: July 22, 1970 PrimaryExaminerRichard C. Queisser Assistant Examiner-C. E. Snee, lll [21]Appl' 57342 Attorney-G. R. Harris and T. A. Zalenski 52 us. Cl. ..73/23,23/232 E, 23/254 E, [571 ABSTRACT 23/255 E The time required forcontinuous gas analyzers of blast fur- [51] Int. Cl. ..'....G01n 31/00nace top gas and the like to reach steady-state conditions after [58]Field of Search ..73/23, 23.1, 27, 19, 25, 26, an interruption offurnace operation is greatly reduced by a process including drying thestream of gas analyzed and sealing a gas containing the analyzedconstituents within the gas analysis apparatus during the period ofinterruption.

8 Claims, No Drawings METHOD OF OPERATING GAS ANALYSIS APPARATUS It iscommon in chemical processes to analyze continuously the off gases froma chemical reaction in order to determine its progress. An example ofsuch process is that of smelting iron in a blast furnace, whichoperation ordinarily is carried on continuously 24 hours a day, 7 days aweek. The furnace is charged continuously and blast is blown through thetuyeres continuously, though at reduced pressure when the furnace isbeing tapped, and iron and slag are tapped out at intervals of a fewhours. The progress of reactions in the furnace can be gauged by therelative amounts of carbon monoxide and carbon dioxide in the top gas,and those compounds, together with the hydrogen content of the top gas,are conventionally determined by apparatus which continuously samplesthe top gas and continuously analyzes those constituents. My inventionto be described is particularly will adapted to the continuous analysisof blast furnace top gas and will be described hereinafter in thatcontext, but it is likewise adapted to the continuous analysis of otherconstituents in off gases from other chemical reactions.

The methods of continuously analyzing the constituents of blast furnacetop gas above mentioned and the apparatus used therefor are well known,and in themselves form no part of my invention. It is conventional toseparate the gross solids from the top gases in an apparatus known as adust catcher. The gas sample to be analyzed is taken from a pointdownstream from this apparatus and the gas is filtered throughsuccessive filters designed to remove solid particles larger than about0.07 microns in size. The gas so filtered is then passed throughmeasuring units adapted to measure continuously and to record thecontents of the constituents of interest. As had been mentioned, thoseof greatest interest are C0, C and H Occasionally, methane, CH is alsomeasured. The C0, C0 and CH are usually measured by passing the gasthrough separated infrared analyzing cells. There infrared radiation istransmitted through the gas stream to a detector. Each of theconstituents of the top gas above mentioned preferentially absorbsinfrared radiation at specific wave lengths in proportion to itsconcentration in the gas stream. In one type of analyzer, the separateradiation detectors are thermopiles which convert the radiation to whicheach is sensitive into an electrical signal.

Hydrogen does not absorb infrared radiation and is generally measured bya thermal conductivity apparatus. The analyzer uses a Wheatstone bridgecircuit with a measuring and reference arm. In each of those arms is acell with a filament. The filament is heated to a high temperature bythe current passed through it. Gas to be analyzed flows through themeasuring cell, and the reference cell is filled with a reference gas.The heat loss from the filament in the measuring cell de pends on thethermal conductivity of the gas constituent flowing through it. Thisheat loss and change in electrical resistance of the filament causedthereby is proportional to the H content of the gas. The bridgeapparatus continuously compares the electrical resistance of thefilaments in the measuring and the reference cells and its output is anelectrical signal proportional to the concentration of H in the gasstream.

The sample gas is conducted into the gas analysis apparatus brieflydescribed above and conventionally is exhausted therefrom into theatmosphere. For calibration purposes, gases of known concentrations ofthe constituents measured are provided, usually in pressure cylinders,and at intervals are connected to the gas analysis apparatus in place ofthe sample gas. The gas analysis apparatus when connected either to thesample gas or calibrating gas is sealed off from the atmosphere, exceptat its exhaust end. In operation, the pressure of the sample orcalibrating gas leaving the apparatus is above atmospheric pressure. Theresistance to flow of the filters and other apparatus in the stream ofsample gas is overcome by suitable motor driven pumps.

Normally, a blast furnace operates under continuous blast, as has beenmentioned, and so continuously evolves top gas. However, it is sometimesnecessary to interrupt operations so that the evolution of top gasceases. When this happens, the

gas analysis apparatus is cut off from the sample stream, usually byshutting a valve in the sample pipeline. The pumps connected therewithare turned off. Conventionally, this allows atmospheric air to enter theanalyzer through the exhaust pipe.

When the furnace operation recommences, it is conventional to check thecalibration of the gas analysis apparatus before reconnecting it to thesampling stream. This is done by passing a calibrating gas through theapparatus. Normally, the gas analysis apparatus will give correctreadings in a period of time of about 2 minutes and it is then connectedto the sampling stream. When the gas analysis apparatus is checkedagainst the calibrating gas in regular operation, it normally givescorrect readings in about the same 2 minute period of time.

It sometimes happens, however, that after an interruption of furnaceoperation, the gas analysis apparatus requires a period of time as longas an hour to come up to equilibrium. The gas analysis apparatus willinitially give CO and CO content readings lower than the known contentsof those compounds in the calibrating gas. The H reading, on the otherhand, will be above the H content of the calibrating gas and will slowlyreduce to the correct value. During such periods of time, the gasanalysis apparatus cannot be used and the operator has no check on theoperation of the furnace. This condition seems to be associated withfurnaces nearing the ends of their campaign.

It is an object of my invention to provide a process of analyzing offgas from a chemical operation which obviates the delays mentioned in thepreceding paragraph. Several embodiments of my invention presentlypreferred by me will appear in the description of my invention whichfollows:

Those skilled in the art of continuous gas analysis have observed thatin spite of the filters employed to remove all solid particles from thegas stream being analyzed, a thin layer of fine dust builds up insidethe gas analysis apparatus. Deposits of this sort are not unknown inapparatus used in mill environments and as a practical matter, seem tobe impossible to prevent. I have discovered that this dust sometimesincludes a compound which acts like silica gel and appears to be a formof activated silica. This compound adsorbs CO and CO from a gas streamcontaining those constituents until it is saturated. If it is thenexposed to a gas stream or an atmosphere which does not contain thoseconstituents, or which contains moisture, these are desorbed. Duringnormal analysis of dry sample gas in gas analysis apparatus in whichthis dust is deposited the adsorbent compound remains saturated with COand CO and the analyzer indicates correctly any change of theconcentration of those constituents in the gas stream. If the gasanalysis apparatus is then switched to a calibrating gas it readscorrectly in a period of time of 2 minutes or so. If, however,atmospheric air or other gas not containing CO and CO gets into the gasanalysis apparatus, the activated silica desorbs its CO and CO to thatgas. When a gas containing those constituents, such as a calibratinggas, is reintroduced into the gas analysis apparatus, the activatedsilica particles adsorb those constituents from the gas stream untilthey are again saturated.

I have also discovered that the activated silica in the dust adsorbsmoisture, in preference to CO and CO from a moisture-containing gaspassed over it and desorbs that moisture to a dry gas. Blast furnace gasnormally contains some moisture and so saturates the activated silicaparticles with moisture during normal continuous gas analysis. If, whenblast furnace operation is interrupted, the gas analysis apparatus isallowed to fill with atmospheric air, the activated silica particles donot desorb that moisture because atmospheric air normally containsmoisture. Then, if a dry calibrating gas is introduced into the gasanalysis apparatus when the blast furnace resumes operation, themoisture on the activated silica particles is desorbed into the gasstream. The dry solids then adsorb CO and CO from the calibrating gasand the H content, as read by the gas analyzer, shows a higher valuethan the actual concentration of that element in the calibrating gasuntil the particles are saturated with CO and CO In one embodiment of myinvention, I clean out all deposited dust from the gas analysisapparatus before it is put back into use after an interruption ofoperation of the blast furnace. While this embodiment of my process iseffective, it is tedious to carry out because it requires dismantlingand reassembly of the gas analysis apparatus. All dust must be removedfrom all the pipes, valves, pumps, filters, absorption cells,conductivity cells and other apparatus components, by hand.

In another embodiment of my invention, I maintain CO and CO equilibriumby sealing off the gas analysis apparatus, both at entry and dischargeends, from all wet gases and from those not containing CO and CO duringperiods of interruption of blast furnace operation. This is accomplishedeither by passing a substitute gas containing CO and CO through the gasanalysis apparatus during the entire time it is disconnected from theblast furnace gas sampling stream or by closing off the gas analysisapparatus from the outside atmosphere at both entry and discharge endsso that the CO and CO containing gas in the gas analysis apparatus atthe time of interruption is trapped in that apparatus.

In either case mentioned above, the adsorbent silica particles aremaintained in a CO and CO saturated condition until regular operationsresume. The substitute gas can be the calibrating gas or any gascontaining CO and C The apparatus is closed off by providing it with avalve at its discharge end as well as at its entry end and closing thedischarge valve before closing the entry valve.

To further maintain equilibrium, I dry the sample gas, first byconventional means to a dew point of about 40 F. (0.8 percent moisture),then to less than 500 parts per million of moisture with a desiccant,such as calcium sulfate. The calibrating gas is dry; therefore, thesample gas must also be dry. If moisture is present in the gas, thesilica gel adsorbs moisture preferentially and desorbs the CO and CO Adry gas then desorbs the moisture but the dry solids again adsorb CO andCO until equilibrium is reached. By dry" gas I mean a gas with amoisture content of about 500 parts per million or less.

As I have mentioned, the calibration difficulties usually appear when ablast furnace is nearing the end of a campaign. A furnace campaign endswhen the furnace lining is badly worn. it is quite likely that theactivated silica particles carried over by the blast furnace gas comefrom the disintegrating furnace lining.

I claim:

1. The method of continuously analyzing off gas from a chemical reactioncarried on continuously subject to interruptions, comprising passing itthrough continuous gas analysis apparatus containing unwanted materialwhich adsorbs a constituent to be analyzed from the off gas and desorbsit to gases not containing that constituent, preventing entrance intothe apparatus of gases not containing that constituent, sealing off thegas analysis apparatus from gases not containing that constituent duringperiods of interruption of the chemical reaction, and again passing theoff gas through the gas analysis apparatus when the interruption ceases.

2. The method of claim 1 in which the gas analysis apparatus is sealedoff by passing therethrough a substitute gas containing thatconstituent.

3. The method of claim 1 in which the gas analysis apparatus is sealedoff from all external gases so as to maintain within it the identicalgas which was present there prior to interruption.

4. The method of claim 1 in which the off gas is blast furnace top gas,and the gas analysis apparatus is adapted and adjusted to analyze theoff gas for at least one of its constituents carbon monoxide and carbondioxide.

5. The method of claim 1 in which the off gas is blast furnace top gas,the gas analysis apparatus is adapted and adjusted to analyze the offgas for at least one of its constituents carbon monoxide and carbondlOXlde, and for hydrogen, and

including the step of drying the off gas before introducing it into thegas analysis apparatus.

6. The method of claim 5 in which the off gas is dried to a moisturecontent of not more than about 500 parts per million.

7. The method of claim 1 in which the unwanted material is fine solidparticles carried by the off gas and lodged in the gas analysisapparatus.

8. The method of claim 7 in which the fine solid particles comprisesurface active silica.

* IIK

1. The method of continuously analyzing off gas from a chemical reactioncarried on continuously subject to interruptions, comprising passing itthrough continuous gas analysis apparatus containing unwanted materialwhich adsorbs a constituent to be analyzed from the off gas and desorbsit to gases not containing that constituent, preventing entrance intothe apparatus of gases not containing that constituent, sealing off thegas analysis apparatus from gases not containing that constituent duringperiods of interruption of the chemical reaction, and again passing theoff gas through the gas analysis apparatus when the interruption ceases.2. The method of claim 1 in which the gas analysis apparatus is sealedoff by passing therethrough a substitute gas containing thatconstituent.
 3. The method of claim 1 in which the gas analysisapparatus is sealed off from all external gases so as to maintain withinit the identical gas which was present there prior to interruption. 4.The method of claim 1 in which the off gas is blast furnace top gas, andthe gas analysis apparatus is adapted and adjusted to analyze the offgas for at least one of its constituents carbon monoxide and carbondioxide.
 5. The method of claim 1 in which the off gas is blast furnacetop gas, the gas analysis apparatus is adapted and adjusted to analyzethe off gas for at least one of its constituents carbon monoxide andcarbon dioxide, and for hydrogen, and including the step of drying theoff gas before introducing it into the gas analysis apparatus.
 6. Themethod of claim 5 in which the off gas is dried to a moisture content ofnot more than about 500 parts per million.
 7. The method of claim 1 inwhich the unwanted material is fine solid particles carried by the offgas and lodged in the gas analysis apparatus.
 8. The method of claim 7in which the fine solid particles comprise surface active silica.